Today’s Google Doodle celebrates the life of the nineteenth-century, French physicist Léon Foucault, and features one of his most impressive inventions: the Foucault pendulum.

Back in the 19th century, it was well known that the earth rotated on its axis, but scientists had struggled to find a simple way of demonstrating this concept. Some particularly ambitious researchers tried dropping weights from high altitudes, or even launching a cannon balls vertically upwards and hoping the earth rotated sufficiently while the projectile was airborne that the ball’s launch point and landing point would deviate in a measurable fashion.

Luckily, before anyone could get seriously injured by these types of experiments, Foucault came to the rescue. He devised a stunningly-elegant test using a multi-directional pendulum. The pendulum would be released over a thin layer of sand, at such a height that the pendulum’s bob would barely graze the top of the sand, in a path that looks like this:

DemonDeLuxe / WikiMedia

While it might look like the pendulum is changing direction while the surface stays static, it’s really the other way around. The earth is spinning, and that movement is ever-so-slowly moving the sand placed under the pendulum’s path.

Today’s Google doodle demonstrates two aspects of this phenomenon. First, it shows how the earth moves under the pendulum. Second, it shows how the speed of the pendulum’s apparent movement depends on where the experiment is held. (The further from the equator the pendulum is placed, the faster it will seem to move around a circle. The closer it is to the equator, the less it appears to move at all, because both the device’s bob and the earth are rotating on virtually the same plane.)

If this all seems a little complicated, well, that’s what the doodle is for! Try messing with the time of day and different locations to get a feel for how the experiment works. The doodle automatically detects where you are currently located, but you can alter that by moving the slider above the globe icon. Want to learn more? This Dartmouth professor does a great job explaining the science behind Foucault’s invention.

It’s also fun to watch one of these pendulums in real life. The United Nation’s headquarters has a permanent display, as do various museums around the world. Here’s a neat demonstration from the Oregon Convention Center: